Various catalysts have been proposed for fluorination processes which substitute fluorine atoms for chlorine. Frequently, these catalysts are oxides or halides of chromium, aluminum, cobalt, iron, titanium, nickel, copper, palladium or zirconium; which are unsupported or supported on active carbons or alumina.
The chromium catalysts are divided into three groups:
catalysts of chromium fluoride PA1 catalysts of crhomium oxyfluoride PA1 catalysts of chromium oxide PA1 (a) formation of a sol or a semi-colloidal suspension of hydroxide of chromium, PA1 (b) gelling of the sol, in the form of microspheres, at an elevated temperature in an organic solvent that is non-miscible or only partially miscible in water, PA1 (c) washing the microspheres with ammonia and water to eliminate the majority of anions, PA1 (d) drying the microspheres at a temperature of, advantageously, about 130.degree. C., PA1 (e) thermally treating the microspheres at a temperature between about 200.degree. and 500.degree. C., in order to develop their catalytic activity.
Chromium oxide catalysts are described, for example, in British Pat. No. 896,068 and U.S. Pat. No. 3,157,707. These catalysts are obtained by the dehydration of chromium hydroxide or by the reduction of chromium trioxide deposited on activated alumina.
French Patent No. 1,369,782 discloses catalysts comprising non-supported chromium oxides obtained by the dehydration of chromium oxide.
U.S. Pat. No. 3,258,500 describes a catalyst having a anhydrous non-supported trivalent chromium base that is prepared by the reduction of chromium trioxide by ethanol and activated by heating in an inert atmosphere of 400.degree.-600.degree. C.
Other patents such as Japanese Applications Nos. 70.116696 and 74.131610, teach the reduction of chromium trioxide by adlehydes or hydrazine.
Catalysts having a black chromium oxide base, obtained by the thermal decomposition of chromium trioxide, chromic hydroxide or chromic carbonate in the presence of air or oxygen, are disclosed in French Patent No. 1,358,997.
U.S. Pat. No. 3,978,145 teaches that the chromium oxide catalysts previously prepared are not amorphous, but have the crystalline structure of an orthorhombic .gamma.-CrOOH. The disclosure proposes catalysts having improved utility, comprising an hexagonal form of this hydroxide-oxide of chromium.
All of the chromium oxide catalysts of the prior art are more or less suitable for gaseous phase fluorination of aliphatic chlorinated derivatives in fixed bed reactor systems. In fluidized bed reactors, which require particles of regular form and homogeneous granulometry, the prior art catalysts are poorly adapted to fluorination processes. Simple grinding of the catalysts, followed by sifting for the selection of suitable-sized particles provides particles of irregular form and leads to a significant loss of the catalyst.
A further disadvantage of known chromium oxide catalysts is a weak resistance to crystallization that ultimately contributes to a shortening of their effective life span. Most of these catalysts yield a ratio relatively elevated in asymmetric isomers when they are used for the preparation of tetrachlorodifluoroethane, trichlorotrifluoroethane and dichlorotetrafluoroethane. Ideally, the content of these asymmetric isomers should be as low as possible since they are more sensitive to hydrolysis and are, therefore, more corrosive.